The Redis SETNX command sets a key to a given value, but only if the key doesn’t already exist. If the key already holds a value, no operation is performed.

Note: The SET command can do the same thing, and it’s possible that the SETNX command may be deprecated at some point. Therefore, it’s probably a good idea to use SET instead of SETNX if possible.

Syntax

The syntax for SETNX goes like this:

SETNX key value

Where key is the key to set, and value is the value to set.

It’s basically the same as doing this:

SET key value NX

Example

Here’s a simple example to demonstrate:

SETNX lunch "Pizza"

Result:

(integer) 1

The value was set because the key didn’t already exist. In this case, the return value is an integer reply of 1.

When the Key Already Exists

Here’s what happens when the key already exists:

SETNX lunch "Greek Salad"

Result:

(integer) 0

This time the value wasn’t set and we got an integer reply of 0.

Just to be sure, let’s check the value of the lunch key:

GET lunch

Result:

"Pizza"

It’s value is still the same that we assigned in the first example.